1. Field of the Invention
The present invention relates to a magnetooptical recording medium for effecting information recording and reproduction with laser light, a method for producing the recording medium, and a method for recording or reproducing information in or from the recording medium.
2. Related Background Art
For rewritable high-density information recording, keen interest is being paid to the magnetooptical recording medium in which information is recorded by forming a magnetic domain in a thin magnetic film with thermal energy of a semiconductor laser and is reproduced by the magnetooptical effect, and, more recently, there is increasing a demand for increasing the recording density of such a magnetooptical recording medium to attain a further increased recording capacity.
The linear recording density of an optical disk, such as the magnetooptical recording medium, is principally determined by the S/N ratio of the reproducing layer, and is significantly dependent on the frequency of the signal bit train, the wavelength of the laser employed in the reproducing optical system, and the numerical aperture of the objective lens.
More specifically, once the wavelength .lambda. of the laser of the reproducing optical system and the numerical aperture NA of the objective lens are given, the bit frequency f of the detection limit is determined by the following equation: EQU f=.lambda./2NA
On the other hand, the track density is principally limited by the crosstalk, which is principally determined by the distribution (profile) of the laser beam on the medium surface and can also be represented by a function of (.lambda./2NA) as in the case of the bit frequency mentioned above. Consequently, for achieving a higher density in the conventional optical disk, it is necessary to shorten the wavelength of the laser in the reproducing optical system and to increase the numerical aperture NA of the objective lens.
However, there are certain limits in such reduction of the wavelength of the laser and increase in the numerical aperture.